Waveform transient measuring circuit and method

ABSTRACT

A waveform transient measuring circuit consisting of two transmission lines and a plurality of spaced coincidence detectors connecting the two lines. The two transmission lines have different propagation rates so that a first signal, introduced into the slower delay line is overtaken by a second signal introduced at a later time into the faster transmission line. The spaced coincidence detectors determine the exact point along the transmission lines at which predetermined corresponding amplitude conditions occur. By calibrating the transmission lines and precisely positioning the detectors, a very accurate timeamplitude measurement is obtained.

United States Patent Furois Aug. 29, 1972 [54] WAVEFORM TRANSIENTMEASURING CIRCUIT AND METHOD Inventor: Philippe C. Furois, Fishkill,N.Y.

Assignee: International Business Machines Corporation, Armonk, NY.

May 6, 1970 35,050

Filed:

Appl. No.:

US. Cl. ..324/188, 333/70 S, 333/84 M Int. Cl ..G04f 9/00, H03h 9/00,H01p 3/00 Field of Search ..324/188, 185; 328/129, 110,

References Cited UNITED STATES PATENTS 8/1965 Bray et a1. ..324/1ss2/1968 Van Zurk ..324/1ss 5/1960 Guillon et a1 ..32s/1 1o 3,418,64112/1968 Fyfeetal. ..333/34X Primary Examiner-Alfred E. SmithAttorneyHanifin and Jancin and Theodore E. Galanthay ABSTRACT 12 Claims,10 Drawing Figures c v LINE 544 315 266 251 221: IMPEDANCE\ (OHMS) -50.052.6 55.6 51.2 6

PATENTEU I973 3.688.194 sum 3 or 3 Wv FIG. 8

FIG. 10

WAVEFORM TRANSIENT MEASURING CIRCUIT AND METHOD BACKGROUND OF THEINVENTION 1. Field of the Invention The invention relates towaveform-transient measuring circuits. More specifically, this inventionrelates to a circuit for determining the transient potential of awaveform at a particular instant of time with a resolution in the orderof a few pico-seconds.

2. Description of the Prior Art An increase in the speed of electronicswitching circuits, particular electronic switching circuits of the typeused in digital computers, has obsoleted presently known techniques fortesting such circuits. Such high speed electronic switching circuitsrely on precise timing for accuracy. When a large number of thesecircuits are interconnected to form a more complex network, even a smalldiscrepancy in the operation of any one circuit can adversely affect thetime coincidence needed at the input of a subsequent circuit. It is,therefore, necessary to test the circuits by accurately measuring theirtime-voltage characteristics.

High speed digital switching circuits are most commonly fabricated inintegrated form and it is known to fabricate a plurality of circuits ona single semiconductor chip. The characteristics of such integratedcircuits frequently cannot be measured by a repetitive testing systembecause of potential overheating. The characteristics of the circuitmust therefore be measured quickly and on a single shot basis.

SUMMARY OF THE INVENTION Accordingly, it is an object of this inventionto accurately measure the time-voltage characteristics of a high speeddigital switching circuit.

,It is another object of this invention to perform the time-voltagemeasurement on a single shot basis.

In accordance with one aspect of the invention, two

transmission lines having different propagation rates are provided. Thefirst of these transmission lines is a strip-line having parallel groundplanes spaced on either side. The second of the two transmission linesis either a micro-strip or micro-line having a ground plane spaced ononly one side thereof. The strip-line having two ground planes has aninherently slower propagation rate than the micro-strip or micro-linewhich only has one ground plane. Coincidence detectors are attached tocorresponding points along both the strip-line and-the micro-line. Withsuch an arrangement, a strobe impulse is inserted into one end of thestrip-line and a signal pulse is inserted into the same end of themicroline. The detectors are biased to switch their state when thepotential of the micro-line has reached the corresponding potential ofthe strip-line. Therefore, as the signal in the micro-line sweeps thestrobe impulse, a detector will be activated at a point where the signalpulse reaches the predetermined coincidence potential. From that pointon, of course, the signal pulse is ahead of the strobe impulse and allsubsequent detectors will also be activated. By appropriate calibrationas described in greater detail hereinbelow, the precise instant of timeat which the signal pulse reaches the predetermined potential isobtained by observing the first activated detector. In accordance withthis embodiment, each of the transmission lines is constructed with astepwise decrease in width in order to maintain a constant impedance.The particular stepwise decrease used by way of example in thisspecification is for equal spacing of the detectors.

In accordance with another aspect of the invention, transmission lineshaving the same rate of propagation are provided. With this embodiment,the strobe and input signals are introduced at opposite ends of the twotransmission lines and a detector is activated at a point along thetransmission lines where a coincident potential is reached.

The foregoing and other objects, features and advantages of thisinvention will be apparent from the following more particulardescription of the preferred embodiments of the invention, asillustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representationof a preferred embodiment of the invention.

FIG. 2 is a series of graphical representations illustrating thecalculated characteristics of the micro-strip and experimentalcharacteristics of the strip-line, micro-strip and micro-line, forpolyguide material.

FIG. 3 is a series of waveform diagrams representing a strobe and signalwaveform.

FIG. 4 is a fragmentary perspective view of a stripline.

FIG. 5 is a perspective view of a micro-strip.

FIG. 6 is a fragmentary perspective view of a stripline and micro-linein a single unitary package.

FIG. 7 is a schematic diagram of an alternate embodirnent.

FIG. 8 is a single tunnel diode detector shown connected to detectpositive-going waveforms.

FIG. 9 is a tunnel diode shown connected to detect negative-goingwaveforms.

FIG. 10 shows tunnel diodes connected to detect both positive-going andnegative-going waveforms.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Refer now to FIG. 1 for adescription of the preferred embodiment. Transmission lines D1 and D2are .terminated to ground through resistors RTl and RT2. Atpredetermined intervals, such as at 1 inch intervals, for example, thedetector circuitry is connected to each transmission line. Since thesetransmission lines are utilized to provide varying degrees of delaytransmission" lines and delay lines are used interchangeably herein.Resistors R1 to R10 are connected to delay lines D1 and resistors R11.and R20 are connected to delay line D2 as shown. Each of these pairs ofresistors are connected to a common point and further connected to atunnel diode. Each tunnel diode is also connected to a source of biasconsisting of a potential source V in series with a biasing loadresistor. This biasing condition normally maintains the tunnel diodes intheir lower state and will determine the voltage level on the unknownsignal at which coincidence occurs. It then requires a combination ofsignals from both delay line D1 and D2 to activate a tunnel diode intoits higher state. Each corresponding point along delay line D1 and D2has its independent detector circuitry. For example, resistors R1 R11,RH, and tunnel diode TDl detect the potential levels at the firstcorresponding points along the delay lines; resistors R2, R12, and RD2and tunnel diode 2 at the second such point, etc. Correspondingresistors such as RI and R11, R2 and R12, and R10 and R20 usually havethe same value of resistance. By way of example, it has been found that1,000 ohms is a convenient design value.

In order to maintain the impedance of the delay lines D1 and D2 at 50ohms (which is also an arbitrarily chosen design value), it has beennecessary to decrease the width of the center conductors as shown inFIG. 1. In the preferred embodiment this tapering has been shown as astepwise decrease in the width of the conductor. The width andcorresponding impedance at each increment are indicated in FIG. 1 by wayof example for the particular dielectric material which is polyguidehaving a thickness of one-eight inch. In order to consistently maintainthe 50 ohm impedance, the terminating resistors RTI and RT2 should eachhave a value of 100 ohms. The values of RLl, RL2, etc. and the value ofpotential source V will vary somewhat with the particular tunnel diodechosen and the biasing of tunnel diodes in this manner is well-known tothose skilled in the art. The output arrow at each of the detectorsindicates an input to any convenient electronic sensor. Such electronicsensors could be simple indicator lights or a computer, depending on thedesired sophistication of the overall system.

Refer now to FIG. 4 which shows a strip-line l adaptable for use asdelay line D1 in FIG. 1. Strip-line comprises a center conductor 16embedded in dielectric material 14 which spaces conductor 16 betweenparallel ground planes 12 and 13. As depicted in FIG. 4, centerconductor 16 has not yet been modified as shown in FIG. 1, the width wbeing subsequently adjusted in accordance with the dimensions indicatedin FIG. 1 at delay line D1 and the thickness 1 is approximately 0.0028inch. The depth d of dielectric material 14 is approximately 0.125 inchthe dielectric constant being approximately 2.41. Such a strip-line hasa propagation rate of approximately 130 picoseconds per inch. The rateis given in inverted form from the normal expression for velocity, inorder to facilitate the numerical expression and mechanical layout. Aformula for calculating propagation time per unit length for astrip-line is as follows:

t,, 1.016 \IF, nanoseconds/feet where E, is the relative dielectricconstant. A formula for calculating propagation time per unit length fora micro-strip is as follows:

r 1.016 I 0.45E,+ 0.67 ns/ft.

Refer now to FIG. 5 which depicts micro-strip 20. Micro-strip 20comprises ground plane 22, dielectric 24 and conductor 26. There is noupper ground plane or covering dielectric as for strip-line 10. FIG. 6shows a micro-line together with a strip-line in a single unitarypackage 30. The micro-line consists of center conductor 26 embedded indielectric 24 with a ground plane 23 on only one side. The strip-line aspreviously shown at FIG. 4 consists of center conductor 16 withdielectric and ground planes on both sides. The micro-line is similar tothe micro-strip of FIG. 6 differing only in that it includes dielectric34 completely surrounding center conductor 26. The micro-strip andmicro-line are also shown prior to the tapering of the width w. Thecenter conductor in each of the FIGS. 4, 5, and 6 is shown prior tobeing modified in order to provide a constant impedance. Themodification providing a constant impedance can be made with a bladewhich cuts the copper material in a fine line, or by chemical etching.It is apparent that such a process could also be automated. Themicro-line has propagation rates similar to that of the micro-strip.Therefore either a micro-line or a micro-strip can be used as the fasterpropagating delay line together with the strip-line which has a slowerrate of propagation.

Refer now to FIG. 7 which shows an alternate embodiment. In thisembodiment, each of the delay lines have the same rate of propagation.The known and unknown waveforms are introduced at opposite ends of eachdelay line and therefore opposite ends of each delay line are terminatedby a resistor to ground. Corresponding components are labeled as in FIG.1, and certain components are therefore left unlabeled to avoid crowdingthe drawing. The values given for the components in the embodiment ofFIG. 1 apply to the embodiment of FIG. 7. The same step-tapering must befollowed as in FIG. 1, except that the two lines are identical i.e.,both are strip-lines, micro-lines or microstrips. This must be donebecause R1 to R10 and R11 to R20 place a resistance in parallel with thecharacteristic impedance of the line. The impedance must be maintainedby decreasing the width of the center conductor.

Refer now to FIGS. 8, 9, and 10 which show tunnel diode detectorsconnected to detect positive-going and negative-going waveforms. FIG. 8depicts the connection of a tunnel diode to detect positive-goingwaveforms. FIG. 9 depicts a tunnel diode detector connected to detectnegative-going waveforms. FIG. 10 depicts tunnel diode detectorsconnected to detect both positive and negative-going waveforms. In theparticular embodiment shown in FIG. 10, the detectors are alternatelyplaced to detect positive and negative-going waveforms. In thisembodiment if only positive-going waveforms are to be detected, thenonly every alternate detector is to be potentially activated. That is,the bias on the detectors is adjusted such that a predeterminedamplitude level of a positive-going waveform is detected only by tunneldiodes connected to the +V source of potential. By analogy, the same istrue if only a negative-going waveform is to be detected. However, if acomplete pulse is to be analyzed, then all the detectors are utilized,thereby determining the time-amplitude characteristics of both thepositive-going and negative-going portions of an unknown wave-form. Thisparticularly useful if it is desired to measure both the rise-time andfall-time of an unknown pulse.

OPERATION In operation, an important aspect of the invention depends onthe difference in propagation rate between a strip-line (or micro-line)or two different transmission lines such as coax or lumped circuits.Such propagation rates are precisely determined in accordance with thepreviously mentioned formulas. Once this difference in propagation rateis determined, it remains a precise and stable condition since the delayline is a passive element. Once the relative propagation speeds aredetermined, detectors are spaced at convenient equal intervals dependingon the resolution desired. The amount of resolution obtained, of course,is also variable by the relative propagation rate of the particulardelay (or transmission) lines.

The attaching of the detectors at 1 inch intervals, for example, causesthe impedance of the parallel delay lines to decrease at that point.Accordingly, it has been found, that the width of the center conductormust be decreased in order to compensate for this and maintain aconstant impedance. This decrease in the width can be a stepwisedecrease in width at the point along the gelay line where the detectors.are attached as shown in The characteristic impedance of a strip-line oflow impedance (50 Q) is provided by the formula:

9X15 Jr. W

Z ohms W/b-t 0.35 and t/b s 0.25

The characteristic impedance of a micro-strip of low impedance (50 Q) isprovided by the formula:

where: W/b 1.20

These formulas are valid near low impedance of approximately 50 ohms andother formulas are valid near higher impedances such as 100 ohms. Forexample, for a strip-line having a high impedance (100 ohms) theimpedance is provided by the formula:

where: t/b s 0.25 and W/b-t s 0.35

d is equal to 0.67 (0.8 w t) However, a discontinuity occurs atapproximately 83 ohms. For this reason, the graph of FIG. 2 wasconstructed from experimental data in order to determine the actualrequired width of the center conductor in each of the delay lines. Thewidth/impedance characteristics of a strip-line, micro-line andmicro-strip as obtained from experimental data is shown in FIG. 2. Alsoshown is the calculated characteristics of a micro-strip in accordancewith the above formula, clearlyillustrating the discontinuity near 83ohms. From the graph of FIG. 2 the actual dimension of the delay line inFIG. 1 are obtained. The actual decrease in impedance is determined byresistors R1, R2, R3, etc. In the present example, each of theseresistors R1, R2, etc. are equal to 1,000 ohms. The line impedancerequired for each step between these resistors is given by thewell-known formula for calculating parallel resistance:

where: Z impedance of previous step z= SO'RIIRL-SO From this formula itis determined that 52.6 ohms in parallel with 1,000 ohms provides animpedance of 50.0 ohms. Entering the graph of FIG. 2. at 52.6 ohmsindicates that the width of the strip-line should be approximately l84mils and the width of the micro-line should be approximately 315 mils,at the point where the first 1,000 ohms is connected. All subsequentwidths of thestrip-line, micro-line (or micro-strip) are obtained inthis manner.

Refer now to FIG. 3 for a detailed description of the operation of thecircuit of FIG. 1. A signal is introduced into the micro-line D2 at Cand propagates to the right as indicated by the arrow. At the same timei=1, a strobe signal is introduced into the strip-line D1 at A. The biason the detectors is selected in accordance with the desired conditionsfor activating the appropriate tunnel diode. As .indicated in FIG. 3,the signal waveform propagates at a faster rate than the strobe pulseuntil time t=t is reached at which time the first tunnel diode isactivated. The signal waveform continues to propagate at a faster ratethan the strobe pulse and will exit the delay line at time t=t,. Oncethe signal waveform has passed he strobe pulse, all subsequent tunneldiodes will also be activated. Accordingly, activation of a given tunneldiode provides the meaningful information concerning the time-amplitudecharacteristics of the signal waveform with respect to the strobesignal. The amplitude of the strobe signal is fixed. The requirement forthe proper functionning of the coincidence detector is that it cannot beactivated with only one signal present. Only the bias on the detectorwill determine what level on the incoming signal will cause coincidenceto occur. With high bias on the detector, the coincidence will be at alow level on the signal with. low bias, the concidence will be at a highlevel on the signal. By determining which level on the signal must bedetected, a time position is known with respect to the time position ofthe-strobe. This is done I by selecting the bias on the detectors.

In order to determine the rise-time of a signal waveform, two sets ofdelay lines as shown in FIG. 1 are used. The bias on the detectors forone line are adjusted to have coincidence. at the 10 percent of thesignal amplitude and the bias on the detectors of the second line areadjusted to have coincidence at the percent of the signal amplitude. Thefirst set is used to determine the 10 percent point of the signal pulseand the second set is used to determine the 90 percent point. Bycomparing the time duration required to reach the 10 percent point withthat required to reach the 90 percent point, the very accurateindication of the rise-time of the waveform is obtained.

The embodiment of FIG. 7 operates in a manner similar to that of FIG.1.'A noteworthy difference is that the two waveforms propagate at thesame rate and must therefore be introduced at opposite ends of the delaylines. The resolution of this alternate embodiment is not as flexible oraccurate as that of the preferred embodiment. The actual physicalstructure becomes a constraint.

In FIG. 7 the resolution is determined by the actual spacing of thedetectors on the line (not the difference of propagation rates orspacing as in FIG. 1). If the propagation velocity is z 33pico-seconds/cm., the resolution of the system will be at best 66pico-seconds. For a resolution of pico-seconds the physical position ofthe detectors would be around one-eighth inch which is beyond mechnicalreality at this time with size of components.

In conclusion, there has been described a waveform transient measuringcircuit consisting of two transmission lines and a plurality ofevenly'spaced coincidence detectors connecting the two lines. Thepreferred embodiment relies for the accuracy of resolution on thedifference in propagation rates between the two transmission lines. Eachof the transmission lines is constructed to maintain a constantimpedance throughout its length. There has also been described a methodfor determining the time-amplitude characteristics of an unknownwaveform to a very high degree of accuracy.

While the invention has been particularly shown and described .withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:

1. Apparatus for determining the time-amplitude characteristics of anunknown waveform comprising:

a first transmission line propagating a known waveform at a firstpropagation rate, said first transmission line being a strip-line;

a second transmission line propagating said u'nknown Y waveform at asecond propagation rate, said second transmission line being amicro-line having a faster propagation rate than said first transmissionline, the unknown waveform thereby overtak ing the known waveform; and

a plurality of detectors connected to corresponding points along thelength of each said transmission line for detecting a predeterminedrelationship between said known and unknown waveforms therebydetermining the time-amplitude charac teristics of said unknownwaveform.

2. Apparatus as in claim 1 wherein:

each said transmission line has its center conductor width decreasedalong its length thereby maintaining a constant impedance.

3. Apparatus as in claim 2 wherein:

the decrease in width is stepwise at each said corresponding point alongthe length of each said transmission line where said detectors areconnected.

4. Apparatus as in claim 1 wherein:

the waveform in the strip-line propagates at 130 pico-seconds per inch;

the waveform in the micro-line propagates at 112 pica-seconds per inch;

the discriminators are connected at 1 inch intervals along the length ofeach said transmission line;

whereby the resolution for detecting the predetermined relationshipbetween said known and unknown waveform is approximately 18pico-seconds.

5. Apparatus as in claim 4 wherein:

the spacing of the detectors is less than 1 inch and the time resolutionis greater than 18 picoseconds.

6. Apparatus as in claim 4 wherein:

the spacing is greater than 1 inch and the time resolution is less than18 pico-seconds.

7. Apparatus as in claim 4 wherein:

the difference in the propagation rate between the two transmissionlines is less than approximately 18 pico-seconds per inch whereby theresolution for detecting the predetermined relationship between saidknown and unknown waveform is greater than 18 pico-seconds per inch oftransmission line.

8. Apparatus as in claim 1 wherein:

the plurality of detectors comprise tunnel diodes.

9. Apparatus as in claim 8 wherein:

any one of the said tunnel diodes is switched from one state to theother in response to predetermined corresponding potential levels of theknown and unknown waveforms.

10. Apparatus for determining the time-amplitude characteristics of anunknown waveform comprising:

a first transmission line propagating a known waveform at a firstpropagation rate;

a second transmission line propagating said unknown waveform at a secondpropagation rate; and

a plurality of detectors connected to corresponding points along thelength of each said transmission line for detecting a predeterminedrelationship between said known and unknown waveform thereby determiningthe time-amplitude characteristics of said unknown waveform, saidplurality of detectors comprising detectors for detecting apredetermined relationship between positivegoing known and unknownwaveforms, and detectors for detecting a predetermined relationshipbetween negative-going known and unknown waveforms, thereby determiningthe time-amplitude characteristics of both positive going andnegative-going unknown waveforms.

11. The method of measuring the rise-time of an unknown waveformcomprising the steps of:

propagating a reference waveform along a first stripline;

propagating the unknown waveform along a first micro-line; Y

detecting coincidence between said reference waveform and said unknownwaveform for determining a first amplitude level of said unknownwaveform, said first level used as the lower level for computingrise-time;

propagating a second reference waveform along a second strip-line;

propagating said unknown waveform along'a second micro-line;

detecting coincidence between said second reference wave-form and saidunknown waveform for determining a second amplitude level of saidunknown waveform, said second level being the upper level for computingrise-time;

comparing the time of occurrence of the lower amplitude level and theupper amplitude level, thereby obtaining an indication of the rise-timeof said unknown waveform.

12. The method of claim 11 for measuring the falltime of an unknownwaveform wherein negative-going pulses are detected such that the stepof comparing comprises:

comparing the time of occurrence of the upper amplitude level with thelower amplitude level, thereby obtaining an indication of the fall-timeof .said unknown waveform.

1. Apparatus for determining the time-amplitude characteristics of anunknown waveform comprising: a first transmission line propagating aknown waveform at a first propagation rate, said first transmission linebeing a strip-line; a second transmission line propagating said unknownwaveform at a second propagation rate, said second transmission linebeing a micro-line having a faster propagation rate than said firsttransmission line, the unknown waveform thereby overtaking the knownwaveform; and a plurality of detectors connected to corresponding pointsalong the length of each said transmission line for detecting apredetermined relationship between said known and unknown waveformsthereby determining the time-amplitude characteristics of said unknownwaveform.
 2. Apparatus as in claim 1 wherein: each said transmissionline has its center conductor width decreased along its length therebymaintaining a constant impedance.
 3. Apparatus as in claim 2 wherein:the decrease in width is stepwise at each said corresponding point alongthe length of each said transmission line where said detectors areconnected.
 4. Apparatus as in claim 1 wherein: the waveform in thestrip-line propagates at 130 pico-seconds per inch; the waveform in themicro-line propagates at 112 pico-seconds per inch; the discriminatorsare connected at 1 inch intervals along the length of each saidtransmission line; whereby the resolution for detecting thepredetermined relationship between said known and unknown waveform isapproximately 18 pico-seconds.
 5. Apparatus as in claim 4 wherein: thespacing of the detectors is less than 1 inch and the time resoLution isgreater than 18 pico-seconds.
 6. Apparatus as in claim 4 wherein: thespacing is greater than 1 inch and the time resolution is less than 18pico-seconds.
 7. Apparatus as in claim 4 wherein: the difference in thepropagation rate between the two transmission lines is less thanapproximately 18 pico-seconds per inch whereby the resolution fordetecting the predetermined relationship between said known and unknownwaveform is greater than 18 pico-seconds per inch of transmission line.8. Apparatus as in claim 1 wherein: the plurality of detectors comprisetunnel diodes.
 9. Apparatus as in claim 8 wherein: any one of the saidtunnel diodes is switched from one state to the other in response topredetermined corresponding potential levels of the known and unknownwaveforms.
 10. Apparatus for determining the time-amplitudecharacteristics of an unknown waveform comprising: a first transmissionline propagating a known waveform at a first propagation rate; a secondtransmission line propagating said unknown waveform at a secondpropagation rate; and a plurality of detectors connected tocorresponding points along the length of each said transmission line fordetecting a predetermined relationship between said known and unknownwaveform thereby determining the time-amplitude characteristics of saidunknown waveform, said plurality of detectors comprising detectors fordetecting a predetermined relationship between positive-going known andunknown waveforms, and detectors for detecting a predeterminedrelationship between negative-going known and unknown waveforms, therebydetermining the time-amplitude characteristics of both positive goingand negative-going unknown waveforms.
 11. The method of measuring therise-time of an unknown waveform comprising the steps of: propagating areference waveform along a first strip-line; propagating the unknownwaveform along a first micro-line; detecting coincidence between saidreference waveform and said unknown waveform for determining a firstamplitude level of said unknown waveform, said first level used as thelower level for computing rise-time; propagating a second referencewaveform along a second strip-line; propagating said unknown waveformalong a second micro-line; detecting coincidence between said secondreference waveform and said unknown waveform for determining a secondamplitude level of said unknown waveform, said second level being theupper level for computing rise-time; comparing the time of occurrence ofthe lower amplitude level and the upper amplitude level, therebyobtaining an indication of the rise-time of said unknown waveform. 12.The method of claim 11 for measuring the fall-time of an unknownwaveform wherein negative-going pulses are detected such that the stepof comparing comprises: comparing the time of occurrence of the upperamplitude level with the lower amplitude level, thereby obtaining anindication of the fall-time of said unknown waveform.